The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the tracking of interventional devices using MRI methods.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles, or "views", in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Intra-operative MR imaging is employed during a medical procedure to assist the doctor in guiding an instrument. For example, during a needle biopsy the MRI system is operated in a real-time mode in which image frames are produced at a high rate so that the doctor can monitor the location of the needle as it is inserted. A locator device such as that described in U.S. Pat. No. 5,622,170 and 5,617,857 may be used to track the location of the instrument and provide coordinate values to the MRI system which enable it to mark the location of the instrument in each reconstructed image. The medical instrument is attached to a handpiece that is manipulated by the physician and whose position is detected by surrounding sensors. For example, the handpiece may emit light from two or more light emitting diodes which is sensed by three stationary cameras.
Tracking devices which employ the MRI system to locate markers in the medical device have also been developed. As described in U.S. Pat. Nos. 5,271,400; 5,307,808; 5,318,025; 5,353,795 and 5,715,822, such tracking systems employ a small coil attached to a catheter or other medical device to be tracked. An MR pulse sequence is performed using the tracking coil to produce transverse magnetization at the location of the tracked device. The location of the tracking coil is determined and is superimposed at the corresponding location in a medical image acquired with the same MRI system.
To accurately locate the tracking coil, position information is obtained in three orthogonal directions that require at least three separate measurement pulse sequences. To correct for errors arising from resonance offset conditions, such as transmitter misadjustment and susceptibility effects, two measurements may be made in each direction with the polarity of the readout gradient reversed in one measurement. This tracking method requires that six separate measurement pulse sequences be performed to acquire the tracking coil location. As disclosed in U.S. Pat. No. 5,353,795, these separate measurements can be reduced to four in number by altering the readout gradients in a Hadamard magnetic resonance tracking sequence.
During a medical procedure in which the location of the tracking coil is periodically acquired and used to update a display, the tracking coil measurement acquisitions are interleaved with image data acquisitions. The rate at which the MR image is updated and the rate at which the medical device position is updated on the image are important system requirements. If the tracking coil position is updated is too slowly, the device will not move smoothly on the display but will instead, jump from one position to the next. Since it requires from three to six separate measurements to update the tracking coil position as described above, the rate at which this update can be performed is limited.